Coaxial waveguide microstructures formed by sequential build processes have been described, for example, in International Application Publication No. WO 00/39854 (WO '854). With reference to FIG. 1A, WO '854 discloses a coaxial waveguide microstructure 100 formed by a sequential build process. The microstructure includes an insulating substrate 102, metallic ground lines 104 formed on the substrate 102 which are disconnected and divided into two parts, metallic supporters 106 formed on the surface of the insulating substrate between the divided ground lines 104, a signal line 108 on the supporters 106 for transmission of the signals, ground walls 110 formed on the ground lines, a ground line 112 formed on the ground walls 110 and an air or vacuous core volume. There are various disadvantages associated with this type of coaxial waveguide structure. For example, the use of metal supporters for supporting the signal line would result in some degree of reflection of the propagated waves and thus create signal interference. Further, the process is not readily scalable to a stacked structure having a plurality of coaxial layer structures due, for example, to the requirement that the metal supporters be connected to the dielectric substrate as a means of support. A multi-layer structure is desirable, for example, in crossovers and for realizing a complex distribution network. In addition, the choice of a substrate material in this known structure is not flexible, but is limited to dielectric materials to achieve similar waveguiding performance. Further, the coaxial waveguide structure cannot be separated from the substrate due to the requisite mechanical connection of the supporters to the substrate.
FIG. 1B illustrates another coaxial waveguide microstructure disclosed in WO '854. The microstructure 114 includes a semiconductor substrate 116, first and second ground lines 118, 120, a dielectric supporter 122, and a signal line 124. Space 126 can be vacuous or air. The semiconductor has a groove, and the first ground line is formed on the surface of the substrate and on the surface of the groove. In this way, the signal line is formed at almost an identical height as that of the surface of the semiconductor substrate, thus allowing easy connection to other connections formed on the semiconductor substrate. This structure, however, also has various disadvantages. For example, the process is not scalable to a multiple coaxial layer structure due to the requirement of a recessed substrate, the geometry of the waveguide structure, and the requirement for a groove on a planar substrate to achieve the requisite planarization. Finally is it not clear that the structure could be self-supporting and thus removed from the substrate, which would be useful, for example, in a stacked coaxial network.
There is thus a need for improved methods of forming coaxial waveguide microstructures which overcome or conspicuously ameliorate one or more of the problems described above with respect to the related art.